A nonlinear or bistable optical device having a very low switching energy is described in U.S. Pat. No. 4,546,244 issued to David A. B. Miller on Oct. 8, 1985. This device has a semiconductor quantum well region which is electrically controlled to change its optical absorption and, in turn, the state of the device. A problem is that optical power must be continuously applied to the device to maintain its state. In one configuration, the device is operated with two input light beams. The power of one input light beam is maintained at a constant level and the power of the other light beam is varied to change the optical absorption and the state of the device. Removal of either beam before the other may cause the device to lose its present state.
Another problem associated with this bistable optical device is that once the power level of the constant power light beam is established, the power range of the variable light beam applied to the device for causing the device to change from one state to another is fixed.